SLF4J Extensions is a library of 3 components: Profiler, Extended logger, Event Logging, Logging added with Java agent.

Here is the source code for SLF4J Extensions You can download its pre-compiled version slf4j-ext-1.7.31.jar at SLF4J Download Website.

⏎ org/slf4j/NDC.java

/**
 * Copyright (c) 2004-2011 QOS.ch
 * All rights reserved.
 *
 * Permission is hereby granted, free  of charge, to any person obtaining
 * a  copy  of this  software  and  associated  documentation files  (the
 * "Software"), to  deal in  the Software without  restriction, including
 * without limitation  the rights to  use, copy, modify,  merge, publish,
 * distribute,  sublicense, and/or sell  copies of  the Software,  and to
 * permit persons to whom the Software  is furnished to do so, subject to
 * the following conditions:
 *
 * The  above  copyright  notice  and  this permission  notice  shall  be
 * included in all copies or substantial portions of the Software.
 *
 * THE  SOFTWARE IS  PROVIDED  "AS  IS", WITHOUT  WARRANTY  OF ANY  KIND,
 * EXPRESS OR  IMPLIED, INCLUDING  BUT NOT LIMITED  TO THE  WARRANTIES OF
 * MERCHANTABILITY,    FITNESS    FOR    A   PARTICULAR    PURPOSE    AND
 * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
 * LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
 * OF CONTRACT, TORT OR OTHERWISE,  ARISING FROM, OUT OF OR IN CONNECTION
 * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 *
 */
package org.slf4j;

import org.slf4j.MDC;

public class NDC {
    public final static String PREFIX = "NDC";

    private static int size() {
        int i = 0;
        while (true) {
            String val = MDC.get(PREFIX + i);
            if (val != null) {
                i++;
            } else {
                break;
            }
        }
        return i;
    }

    public static void push(String val) {
        int next = size();
        MDC.put(PREFIX + next, val);
    }

    public static String pop() {
        int next = size();
        if (next == 0) {
            return "";
        }
        int last = next - 1;
        String key = PREFIX + last;
        String val = MDC.get(key);
        MDC.remove(key);
        return val;
    }

}

⏎ org/slf4j/NDC.java

Or download all of them as a single archive file:

File name: slf4j-ext-1.7.31-sources.jar
File size: 40408 bytes
Release date: 2021-06-17
Download 

⇒ Source Code for SLF4J 1.7.31 Migrator

⇐ Source Code for SLF4J 1.7.31 Bridge to Log4J 1.3

⇑ Downloading and Reviewing SLF4J Packages

⇑⇑ SLF4J - Simple Logging Facade for Java